The application of multidimensional NMR to the study of soil humic substances

Humic substances are the most abundant organic macromolecules in soils, and comprehension of their chemical structure is essential to understanding their role in terrestrial ecosystems. The one-dimensional nuclear magnetic resonance (NMR) spectroscopy techniques now used widely to study humic substances have provided important insight into humic structures, but the complexity of these macromolecules gives rise to resonance signals that are broad and have spectral overlap. This has prevented the definitive functional group assignments necessary for structural determination, Hence, interest has focused on more powerful two-dimensional NMR experiments, such as the homonuclear TOtal Correlation SpectroscopY (TOCSY) and Heteronuclear Multiple Quantum Coherence (HMQC), which were employed in the study of a soil humic acid standard. The purpose of this paper is to outline the potential of these techniques to the study of soil humic structures. The 2-D spectra produced were extremely encouraging, with multitudes of cross-peaks produced from both TOCSY and HMQC experiments. The identification of fatty ester/acid chains and amino acid couplings are given as examples. Results obtained with these NMR experiments indicate substantial improvements in functional group assignment capabilities and the potential for marked progress in the determination of the chemical structure of soil humic substances.